We propose to study the role of joint afferents in promoting medio- lateral stability of the human knee joint Preservation of joint stability is necessary to prevent cartilage damage and osteoarthritis. Joint stability has been attributed to five factors: bone and cartilage contact forces, ligament and capsule stiffness, intrinsic stiffness, intrinsic stiffness and active muscles, and reflexively mediated muscle stiffness.. Reflex action may, in turn, be mediated either by muscle stretch receptors or by periarticular tissue afferents (ligaments and capsule). The latter class of reflexes potentially protects the joint through muscle activation in situations of abnormal varus/valgus loading of the joint. We hypothesize that targeted muscle contractions can be elicited by the application of a mechanical varus/valgus stimulus to the human knee, and that these responses significantly increase the joint's stiffness in the varus/valgus plane. Quantitative measures of individual knee muscle action will be determined in 35 subjects. We will use intramuscular electrodes to selectivity stimulate muscles traversing the knee, and a 6-degree of freedom load cell will measure the movements produced. For the same subject population, a rapid varus/valgus positional perturbation will be applied at the subject knee via a servomotor. During the mechanical perturbation, muscle activity will be recorded and the reflex responses in major knee muscles recorded in all subjects. Based on pilot data, we believe that mechanical perturbations resulting in the stretching of joint ligaments will produce a reflex response in muscle traversing the knee joint, and that these reflexes may be organized so as to compensate selectively for varus or valgus loading, and that these reflexes may be organized so as to compensate selectively for varus and valgus loading. To quality the stiffness contributed by reflexively mediated muscle contractions, a pseudo-random varus/valgus perturbation will be applied to the knees of all 35 subjects before and after nerve blockade. Joint stiffness and damping coefficients will be estimated using linear models. The reflex stiffness mediated by periarticular afferents will be calculated to be the difference in the computed joint stiffness with and without the neural blockade. We believe that stretch receptors in periarticular tissues of the knee joint play a major role in promoting joint media-lateral stability, and we further expect that information about the interaction between neurological and mechanical components will have a major impact on how training may improve overall joint stability. Reprogramming the neural component of the joint by physiotherapy may help substitute the loss of function of ligaments induced by joint laxity and thus limit the rate of OA progression.